This invention relates to a switch unit mounted, for example, on a door panel of a vehicle or the like, and more particularly to an improvement of a switch unit in which a flexible printed circuit member (serving as a wiring circuit member), on which electronic parts are mounted by soldering, and a rubber contact for opening and closing switch contacts on the flexible printed circuit member, are received and held in a casing of a curved structure divided into two (upper and lower) sections, that is, a lower casing and an upper casing.
Various kinds of power-window switch units, installed in the door panels, for opening and closing window pane have been developed, these units includes a wiring circuit member on which electronic parts are mounted by soldering, and a casing which is divided into two (upper and lower) sections, that is, a lower casing and an upper casing for receiving the wiring circuit member received and held therein, and switches for opening and closing circuit contacts within the casing are provided on an upper surface of the upper casing.
In such a switch unit, for example, the surface of the upper casing, on which the switches are provided, is, in many cases, formed into a curved structure in order to enhance the operability of the switches and also to match its design with the interior design. Therefore, in order to enhance the ability of reception into the casing of a curved structure, the wiring circuit member to be received in the switch unit is required to have flexibility so that an undue force will not act on this wiring circuit member arranged in conformity with the curved surface within the casing. In addition to the electric circuits for the power windows, various electric circuits, for example, for adjusting power seats and for controlling courtesy lamps are incorporated in a door panel or the like of the vehicle. Therefore, in order to achieve a high packaging density in a limited space within the door, the wiring circuit member, used in the switch unit, is also required to have a space-saving design.
In order to meet these requirements, there have recently been extensively used flexible printed circuits (hereinafter referred to as xe2x80x9cFPCxe2x80x9d) as a wiring circuit member on which electronic parts are mounted by soldering, and this flexible printed circuit has a wiring circuit printed on a film-like or a sheet-like thin flexible insulative substrate.
FIGS. 10 and 11 show a related switch unit in which an FPC is used as a wiring circuit member on which electronic parts are mounted.
The switch unit 1 includes switches for power windows of a vehicle and other switches and is mounted on a door panel, and is so constructed that the FPC 5 (serving as a wiring circuit member) on which the electronic parts 3 are mounted by soldering, and a rubber contact 7 (serving as a switch mechanism) for opening and closing switch contacts on the FPC 5, are received and held in a casing 11 of a curved structure divided into two (upper and lower) sections, that is, a lower casing 9 and an upper casing 10.
The rubber contact 7 is superposed on an upper surface of the FPC 5, and is the switch mechanism in which each of contact opening-closing conductors 7a for opening and closing the switch contacts 5a formed on the FPC 5 is supported by a bulged rubber wall 7b in spaced relation to the switch contact 5a. 
As shown in FIG. 11, a key top 13 serving as a switch operating member for opening and closing the switch contacts 5a through the rubber contact 7 are provided on the rubber contact 7 in a displaceable manner.
The key top 13, when displaced toward the rubber contact 7, elastically deforms the rubber wall 7b of the rubber contact 7 toward the FPC 5, so that the contact opening-closing conductor 7a on the rubber contact 7 is brought into contact with the witch contact 5a on the FPC 5.
The lower casing 9 and the upper casing 10 have the smoothly-curved structure, and the FPC 5 and the rubber contact 7 are received and held in these casings in a curved condition corresponding to the curved structure of the casings 9 and 10.
However, the whole of the structure is formed into the curved structure, and the switch mechanism is provided on the curved surface. In this form, for example, when the key top 13 is pressed down as indicated by arrow A in FIG. 12 to elastically deform the rubber wall 7b of the rubber contact 7, the rubber wall 7b is subjected to flexural deformation not by an originally-expected vertical load acting in a direction which the rubber wall 7b extends but by a load acting in a direction inclined relative to the direction which the rubber wall 7b extends as indicated by arrow B. As a result, there is encountered a problem that the deformation is concentrated on part of the rubber wall 7, so that the rubber wall 7 is liable to be broken.
Further, there was a possibility of contact failure between the contact opening-closing conductor 7a and the switch contact due to that the displacement position of the contact opening-closing conductor 7a shifts by flexural displacement of the rubber wall 7b. 
As shown in FIG. 13, the portion 15 of the FPC 5, to which the electronic part 3 is soldered, is kept lifted off a bottom wall surface of the lower casing 9 because of the curved structure of the lower casing 9, and therefore, the portion 15 is vibrated by vibrations, developing during the running of the vehicle, and this leads to a possibility that a joint portion, formed by solder, is cracked or separated.
This invention has been made in view of the above circumstances, and an object of the invention is to provide a switch unit in which a casing receiving a flexible printed circuit and a rubber contact can be formed into a curved structure suitable for the switch operability and so on, and further the durability and reliability of the switch unit are excellent.
In order to solve the aforesaid object, the invention is characterized by having the following arrangement.
(1) A switch unit mounted on a curved structure comprising:
a flexible printed circuit including a wiring circuit formed on an insulative substrate, electric parts and switch contacts being mounted on an upper surface of the flexible printed circuit;
a lower casing which holds the flexible printed circuit and includes a bottom wall covering a lower surface of the flexible printed circuit and including a plurality of flat surfaces;
a rubber contact, for opening and closing the switch contacts, superposed on the upper surface of the flexible printed circuit;
an upper casing which includes an upper wall covering the rubber contact, a plurality of switch mounting portions respectively opposed to the plurality of flat surfaces of the bottom wall, and an engagement member through which the upper casing is connected to the lower casing; and
a plurality of key tops, for operating the rubber contact, mounted on the plurality of switch mounting portions, respectively,
wherein the electric parts and the switch contacts are located above the plurality of flat surfaces.
(2) A switch unit according to (1), wherein one of a bending portion and a bending-assisting member for enhancing the flexibility of the flexible printed circuit is provided at a portion of the flexibility of the flexible printed circuit which corresponds to a border of the flat surface.
(3) A switch unit according to (1), wherein the adjacent flat surfaces are connected together by hinge, and the lower casing can be formed into a curved shape corresponding to the upper casing by adjusting the degree of bending the hinge.
(4) A switch unit according to (1), wherein the bottom wall includes the plurality of flat surfaces and a curved surface.
(5) A switch unit according to (2), wherein the bottom wall includes the plurality of flat surfaces and a curved surface, and the border of the flat surface is defined between the flat surface and the curved surface.